1. Field of the Invention
The present invention relates to a master cylinder for use in a brake system of a vehicle such as an automobile.
2. Description of the Prior Art
A master cylinder, especially a tandem type master cylinder which is provided with two oil pressurizing chambers, requires a relatively large mounting space on a vehicle. It is therefore desirable to make the axial length of the master cylinder as short as possible.
There are two types of tandem master cylinders, one in which cup seals are mounted on the piston and another in which cup seals are mounted on the cylinder. The latter is advantageous for shortening the master cylinder, and in this type, the cup seals are usually held by sleeves inserted into the cylinder.
A prior art master cylinder of this type is disclosed in Japanese Patent Public Disclosure (Kokai) No. 210160/87 which corresponds to U.S. Pat. No. 4,685,300. The master cylinder of this type will be explained with reference to FIG. 1. The master cylinder 100 comprises a cylinder 6 consisting of a cylinder body 2 provided with a bottom 1 and a cylinder cap 5 (referred to as "cap" hereafter) threaded in an opening 4 of a cylinder bore 3 of the cylinder body 2, an oil reservoir 7 mounted on the cylinder body 2 and communicating with the interior of the cylinder body 2, first, second and third sleeves 8, 9 and 10, respectively, which are fitted into the cylinder bore 3 of the cylinder 6 in the foregoing order from the bottom 1 side to the opening 4 and fixed in the cylinder by means of the cap 5 being threaded onto the cylinder body 2, primary and secondary pistons 12 and 11 slidably disposed in the cylinder 6 through the sleeves 8, 9 and 10, and springs 13 and 14 disposed in the cylinder 6 for biasing the secondary piston 11 and primary piston 12, respectively, toward the opening 4. An oil pressurizing chamber 15 is defined between the bottom 1 and the secondary piston 11 while another oil pressurizing chamber 16 is defined between the primary and secondary pistons 12, 11. The primary piston 12 extends outside the cylinder 6. In the extending end of the primary piston 12 is formed a recess 17 for receiving therein one end of an output shaft (not shown) connected to a brake pedal or one end of an output shaft of a booster (not shown). As shown, cup seals 22 and 23 are respectively disposed and sandwiched between a shoulder formed in the body 2 and the first sleeve 8 and between the second and third sleeves 9, 10.
When the primary piston 12 receives a driving force from the aforementioned output shaft (not shown), the primary piston 12 is displaced forward (leftward as viewed in FIG. 1) against the spring 14. When a communicating hole 19 formed in the primary piston 12 passes a communicating hole 18 formed in the third sleeve 10 and further passes the rearmost end surface 23a (rightmost as viewed in FIG. 1) of the cup seal 23, the brake oil in the oil pressurizing chamber 16 is pressurized to generate a braking oil pressure. The braking oil pressure thus generated is transmitted through an output port (not shown) communicating with the chamber 16 to the brake cylinder of a brake (not shown), associated with a wheel to operate the brake. At that time, the secondary piston 11 is also displaced in the same direction as the primary piston 12, namely leftward, against the spring 13 and, when a communicating hole 21 formed in the secondary piston 11 passes a communicating hole 20 formed in the first sleeve 8 and further passes the rearmost end surface 22a (rightmost as viewed in FIG. 1) of the cup seal 22, a braking oil pressure is generated in the oil pressurizing chamber 15. The braking oil pressure thus generated is transmitted through an output port (not shown) communicating with the chamber 15 to the brake cylinder of a brake, associated with a wheel, to operate the brake.
When the force acting on the aforementioned output shaft is released, the primary and secondary pistons 12, 11 are both returned backward (rightward as viewed in FIG. 1) by virtue of the forces of the springs 14 and 13, respectively.
When the pressure in the chambers 15 and 16 is reduced during the return stroke, the outer peripheral portions of the cup seals 22 and 23 are deflected by virtue of the pressure difference between the inner and outer sides thereof, so that the oil in the reservoir 7 flows through supplying ports 24 and 25 and through the clearances respectively defined between the outer peripheries of the cup seals 22, 23 and the inner surface of the cylinder bore 3 into the chambers 15 and 16.
A stepped portion 26 is formed in the cylinder bore 3 adjacent the bottom 1 while a stepped portion 27 is formed in the bottom portion of the cap 5. The front or left end of the first sleeve 8 abuts against the stepped portion 26 while the rear or right end of the third sleeve 10 abuts against the stepped portion 27.
A seal member 28 is disposed between the secondary piston 11 and the first and second sleeves 8, 9 while a seal member 29 is disposed between the primary piston 12, the third sleeve 10 and the cap 5.
The prior art master cylinder as explained above suffers, however, from the following problems. Since the first, second and third sleeves 8, 9 and 10 are fitted into the cylinder 6 and fixed in the space defined between the cylinder body 2 and the cap 5 by the cap 5 being threaded into the cylinder body 2 with the first sleeve 8 abutting against the stepped portion 26 of the cylinder body 2 and with the third sleeve 10 abutting against the stepped portion 27 of the cap, these three sleeves 8, 9 and 10 are compressed between the cylinder body 2 and the cap 5, which compression possibly results in the elements of the master cylinder 100 being buckled or becoming cracked. Further, ineffective strokes of the primary piston possibly vary widely. Ineffective stroke T is defined by the stroke of the primary piston 12 between the initial position thereof from which the primary piston 12 is displaced leftward (as viewed in FIG. 1) and the position where braking oil pressure is first generated in the chamber 16. The ineffective stroke T of the master cylinder 100 as shown in FIG. 1 is determined by the following equation. EQU T=(P.sub.1 -P.sub.2)-[(S.sub.1 +S.sub.2 S.sub.3)-(L+.alpha.)](1)
in which
P.sub.1 =distance between the open end of the cylinder body 2 and the bottom of the recess 17, PA0 P.sub.2 =distance between the center line of the communicating hole 19 and the bottom of the recess 17, PA0 S.sub.1 =axial length of the first sleeve 8, PA0 S.sub.2 =axial length of the second sleeve 9, PA0 S.sub.3 =distance between the rearmost end surface of the second sleeve 9 and the rearmost end surface of the cup seal 23, PA0 L=distance between the surface of the shoulder 26 of the cylinder body 2 against which the first sleeve 8 abuts and the opening end surface 2a of the cylinder body, and PA0 .alpha.=compressive height by which the first, second and third sleeves are totally compressed when the cap 5 is threaded into the cylinder body 2.
As shown by the above equation, the ineffective stroke T includes seven parameters which can have respective variations in their dimensions. As a result, the ineffective stroke T can vary widely, resulting in master cylinders with uneven performance characteristics.